This invention relates to a plasma plug used with a high-energy discharge ignition system to produce a "plasma jet". A typical plasma plug comprises a shell for engagement with an internal combustion engine, an annular ground electrode structurally integral with the shell, an insulator mounted in the shell, and a center electrode seated in a bore of the insulator. The center electrode has a firing end which is in spark gap relationship with the ground electrode. The insulator has a stepped cavity including a cavity of relatively large cross-section adjacent the firing end of the center electrode and a cavity of smaller cross-section axially therebeyond.
A "plasma jet" is emitted when successive high voltage and high current pulses ionize and energize the gap between the electrodes of the plasma plug. The plasma jet is a luminous stream of high-temperature, highly ionized gas with projects longitudinally from the cavity of the insulator into the air-fuel mixture of an internal combustion engine in which the plasma plug is installed. The effectiveness of the plasma jet in causing combustion is partly determined by the length and diameter of its luminous region and the turbulence generated thereby. U.S. Pat. No. 2,840,742, granted June 24, 1958 to Clarence J. Watters, discloses a plasma plug wherein the walls of the stepped cavity are coated with a semi-conductive material.